Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butylenes, butadiene, and aromatics such as benzene, toluene, and xylenes. Each of these is a valuable commercial product in its own right. For instance, the olefins may be oligomerized (e.g., to form lubricant basestocks), polymerized (e.g., to form polyethylene, polypropylene, and other plastics), and/or functionalized (e.g., to form acids, alcohols, aldehydes and the like), all of which have well-known intermediate and/or end uses. One thermal cracking process is steam cracking, which involves cracking hydrocarbons at elevated temperatures in the presence of steam or gas mixtures containing steam.
Typically in steam cracking, a hydrocarbon feedstock such as naphtha, gas oil, or other non-resid containing fractions of whole crude oil, which may be obtained, for instance, by distilling or otherwise fractionating whole crude oil, is usually mixed with steam and introduced to a steam cracker. Conventional steam cracking utilizes a pyrolysis furnace that generally has two main sections: a convection section and a radiant section. In the conventional pyrolysis furnace, the hydrocarbon feedstock enters the less severe convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is heated and vaporized by indirect contact with hot flue gas from the radiant section and optionally by direct contact with steam. The vaporized feedstock (and optional steam) mixture is then conveyed (typically through crossover piping) into the radiant section where it is quickly heated, at pressures typically ranging from about 10 to about 50 psig (69 to 345 kPa), to a severe hydrocarbon cracking temperature, such as in the range of from about 1450° F. (788° C.) to about 1650° F. (900° C.), to provide thorough thermal cracking of the feedstream. The resulting products, comprising olefins, leave the pyrolysis furnace for rapid quenching and further downstream separation and processing.
After cracking, the effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety, e.g., saturated, monounsaturated, and polyunsaturated, and can be aliphatic and/or aromatic, as well as significant amounts of molecular hydrogen (H2). The cracked product is then further processed such as in the olefin production plant to produce, as products of the plant, the various separate individual streams of high purity, i.e., hydrogen, the light olefins ethylene, propylene, butylenes, and aromatic compounds, as well as other products such as pyrolysis gasoline and pyrolysis gas oils.
As worldwide demand for light olefins increases and the availability of favorable crude sources is depleted, it becomes necessary to utilize heavier crudes (i.e., those having higher proportions of resid), which requires increased capital investments to process and handle the refining byproducts. It is highly desirable to have processes that can take lower cost, heavier crudes, and produce a higher value product mix of light olefins, more efficiently. However, conventional steam cracking processes are known to be severely limited by fouling when using feedstocks containing high concentrations of resid, which is commonly present in low quality, heavy feeds. Thus, most steam cracking furnaces are limited to processing of higher quality feedstocks which have had substantially all of the resid fraction removed in other refinery processes. Such additional processes increase the cost of the overall process. Likewise, removal of the resid fraction lowers the overall conversion efficiency of the refinery process, since most of the resid fraction is mixed with low value fuel oils, rather than being converted to higher-value materials.
Cracking of heavy hydrocarbon feeds in fluidized cokers has been described in U.S. Pat. No. 3,671,424, incorporated herein by reference, which discloses a two-stage fluid coking process in which the first stage is a transfer line for short contact time and the second is either a transfer line or a fluidized bed.
U.S. Patent Published Patent Application No. 2007/0090018, incorporated herein by reference, discloses integration of hydroprocessing and steam cracking. A feed comprising crude or resid-containing fraction thereof is severely hydrotreated and passed to a steam cracker to obtain an olefins product.
U.S. Pat. No. 4,975,181, incorporated herein by reference, discloses an improved process and apparatus for the pyrolysis of a heavy hydrocarbon feed utilizing a transfer line reactor wherein pyrolysis reaction temperatures are achieved by contact of the heavy hydrocarbon feed with heated solid particles immediately followed by quenching of the pyrolysis gaseous effluent with cooled solid-particles in the transfer line reactor to maximize ethylene production and minimize the effect of secondary reactions.
Other patents of interest related to cracking heavy feeds include U.S. Pat. No. 4,257,871; U.S. Pat. No. 4,065,379; U.S. Pat. No. 4,180,453; U.S. Pat. No. 4,210,520; U.S. Pat. No. 7,097,758; U.S. Pat. No. 7,138,097; U.S. Pat. No. 7,193,123; U.S. Pat. No. 3,487,006; U.S. Pat. No. 3,617,493; U.S. Pat. No. 4,065,379; U.S. Pat. No. 3,898,299; U.S. Pat. No. 5,024,751; U.S. Pat. No. 5,413,702; U.S. Pat. No. 6,210,561; U.S. Pat. No. 7,220,887; U.S. Pat. No. 3,617,493; US 2007/023845; WO 01/66672; WO 2007/117920; U.S. Pat. No. 6,632,351; WO 2009/025640; and WO 2007/117919. Other references of interest include: “Tutorial: Delayed Coking Fundamentals.” P. J. Ellis and C. A. Paul, paper 29a, Topical Conference on Refinery Processing, 1998 Great Lakes Carbon Corporation (which can be downloaded from http://www.coking.com/DECOKTUT.pdf).
There remains in the art a need for new means and processes for economical processing of heavy, resid-containing feeds for the production of olefins, aromatics, and other valuable petrochemical products. All known art previous to this invention, has deficiencies, shortcomings, or undesirable aspects.